HIGH RESOLUTION MAGNETIC SUSCEPTIBILITY AND CONDUCTIVITY ANALYSIS AS A TOOL FOR THE IDENTIFICATION OF DEPOSITIONAL SEQUENCES AND CYCLES IN THE UPPER ORDOVICIAN SALONA AND COBURN FORMATIONS IN CENTRAL PENNSYLVANIA

The Taconic Orogeny brought about the decline of the Great American Carbonate Bank (GACB) of Laurentia. Sedimentary signatures found in Upper Ordovician units (Trenton and Black River Groups) indicate that in a relatively brief time span, significant climatic, oceanographic, and topographic transformations occurred. These are in part attributed to island arc collision along southeastern Laurentia. Given changes in sea-level, climate, and ocean circulation, the GACB was overtopped by siliciclastics. In central Pennsylvania, rock outcrops contain the record of this time interval and are relatively easy to access. Yet due to tectonism, a high-resolution sub-sequence chronology is not well defined nor well-correlated from locality to locality. This preliminary study uses a hand-held Terraplus KT-20 magnetic susceptibility (MS) meter to investigate the Salona-Coburn (SC) interval to aid in the characterization of rock units based on their magnetic and conductive signatures. These units are composed of rhythmic limestone-shale turbidites of varying limestone:shale ratios. These are interpreted to have been deposited below fair and storm weather wave base in proximal to distal ramp foreland basin settings. The meter allows magnetic and conductive signatures to be measured from both field and lab settings. Units composed of clean carbonates, quartzose siltstones, and organic-rich shales show very low MS values, whereas units containing higher amounts of mafic and Fe-Mn minerals (i.e. pyrite, clays such as illites and glauconites, bentonites, etc.) have higher MS values. Initial results demonstrate distinctive trends in MS values for the SC and overlying units. These are complemented by high-order MS oscillations suggestive of systems tracts and high-frequency cycle components. This work is being compared to a previous study that utilized spectral analysis to identify cycle pattern hierarchies based on bed thickness. This study is thus establishing the viability of using MS signatures to resolve existing sequence stratigraphic frameworks, as well as to evaluate higher order cycles that could be correlated outside of PA. These techniques will ultimately create a high-resolution geophysical and lithologic dataset to enable improved cross-basin correlation with regions in New York and the mid-continent.